Encyclopedia
Please note this is a comparison between Version 1 by Yong Zhang and Version 3 by Lindsay Dong.
A new metallurgical strategy was introduced to develop advanced materials with outstanding performance—high-entropy alloys (HEAs). THigh-entroday, HEAs contain five or more multiple principle metallic elements in equal or near-equal atomic percentages. HEAs’ four core effects—high configurational entropy, sluggish diffusion, severe lattice distortion, and the cock-tail effect—are mainly responsible for their various physical and mechanical properties. HEAs present promising properties, such as high strength and fracture toughness at room temperature and high temperatures and have excellent wear resistance, and corrosion resistance, along with high-temperature oxidationpy alloys (HEAs) prefer to form single-phase solid solutions (body-centered cubic (BCC), face-centered cubic (FCC), or hexagonal closed-packed (HCP)) due to their high mixing entropy.
- high-entropy alloys
- BCC structure
- refractory high-entropy alloy
1. Introduction
A new metallurgical strategy was introduced to develop advanced materials with outstanding performance—high-entropy alloys (HEAs). Today, HEAs contain five or more multiple principle metallic elements in equal or near-equal atomic percentages [1]. HEAs’ four core effects [2]—high configurational entropy, sluggish diffusion, severe lattice distortion, and the cock-tail effect—are mainly responsible for their various physical and mechanical properties. HEAs present promising properties, such as high strength and fracture toughness at room temperature [3][4][5][6][3,4,5,6] and high temperatures [2][7][2,7] and have excellent wear resistance [4], and corrosion resistance [8][9][10][11][8,9,10,11], along with high-temperature oxidation [8].
HEAs are more likely to generate a simple solid solution (typically body-centered cubic (BCC), face-centered cubic (FCC), or hexagonal closed-packed (HCP) phases) despite containing many components [12][13][14][15][12,13,14,15]. Up to now, the development of HEAs has mainly gone through three stages: quinary equal-atomic single-phase solid-solution alloys; quaternary or quinary non-equal-atomic multiphase alloys; medium-entropy alloys, high-entropy fibers, high-entropy films and lightweight HEAs (LWHEAs) [16][17][16,17]. HEAs mainly include two categories: refractory elements (such as V, Cr, Ti, Mo, Nb, Ta, W, Zr, and Hf) and commodity metals (such as Cr, Co, Fe, Ni, Mn, and Cu) [18]. HEAs in the first category consist of many refractory elements, so-called refractory high-entropy alloys (RHEAs) [13], and mostly form BCC structures [19][20][19,20]. The second category of HEAs mostly forms FCC structures [21][22][21,22], a combination of FCC and BCC structures [23][24][25][23,24,25] or BCC structures alone [26][27][26,27].
However, FCC structures or “FCC-based” structure HEAs show excellent ductility high plasticity, but its yield strength is not considerable in a large number of studies reported so far [28][29][28,29]. It needs to be strengthened by a series of thermomechanical treatments before industrial application, which requires an additional cost. In contrast to FCC HEAs, BCC HEAs exhibit relatively high intrinsic yield strengths [30][31][30,31]. Besides, RHEA systems have excellent high-temperature mechanical properties but insufficient toughness at room temperature [32][33][32,33]. Developing BCC HEAs with high strength and good ductility is a key to making them potential structural materials in technological applications.
However, it is still weak to regulate the plastic deformation behavior of BCC HEAs via the strengthening methods of FCC HEAs. BCC HEAs can only deal with liquid-solid phase transformation by some processing technology, such as melt purification, homogenization, directional solidification (DS), powder metallurgy (PM) and additive manufacturing (AM).
Nowadays, BCC HEAs have typically been fabricated via simple arc-melting and casting, which has been confirmed to be a highly efficient technique for more than five metals. For instance, the AlCrFeCoNi HEA with a single-phase BCC solid solution exhibits excellent compressive properties of yield stress (1250.96 MPa) and plastic strain (32.7%) [27]. VNbMoTa RHEAs exhibit excellent room-temperature ductility with a fracture strain > 25% and high-temperature strength (compressive yield strength of 811 MPa at 1000 °C) [34]. However, the cast product has some drawbacks including many structural defects, such as voids, porosity, chemical segregation and grain coarsening.
Powder metallurgy (PM), a forming technology that allows significant compositional accuracy can completely prevent chemical segregation, can obtain a homogeneous microstructure, can produce nanocrystalline materials and can develop metal matrix composites [35][36][35,36]. Lightweight RHEAs of CrNbVMo exhibited superior compressive specific yield strengths compared to cast RHEAs at 25 °C and 1000 °C [37]. TiNbTa0.5Zr and TiNbTa0.5ZrAl0.2 RHEAs were successfully prepared without any cracks or fractures by PM, and with Al addition, the compressive peak stress increased from 508 MPa to 603 MPa at 800 °C [38]. The PM process is not only suitable for preparing RHEAs but is also suitable for preparing small and precise components.
In contrast to the conventional process, additive manufacturing (AM) is based on an incremental layer-by-layer fabrication process [39]. Because local process control can be realized in the AM process, it has extremely rapid solidification cooling rates, production with unrivaled design freedom, and shorter production cycles [40][41][40,41]. The AM process can overcome the inherent complexity and achieve the high levels of control required to produce homogeneous bulk alloys. Due to the immanent advantages of AM, it has attracted much attention in the last decade. At present, the process for preparing HEAs mainly includes direct laser deposition (DLD), selective laser melting (SLM) and selective electron beam melting (SEBM) [42][43][44][45][42,43,44,45]. The phases, crystal features, mechanical properties, functionalities and potential applications of these products have been discussed. Because the high density, low ductility at room temperature and poor resistance to oxidation are the main drawbacks of RHEAs, it is a challenge to prepare powder for the AM process.
2. Mechanical Properties of BCC-Structured HEAs at Various Temperatures
2.1. Static Mechanical Properties
2.1.1. Processing by Vacuum Arc-Melting
To provide a relatively systematic overview of the mechanical properties of BCC-structured HEAs, their uniaxial deformation behavior is compared with traditional structural alloys, as shown in Figure 1 [46][56]. BCC-structured HEAs mostly consist of refractory elements, which are generally considered suitable for high-temperature applications. The RHEA systems that have been successfully prepared to date include Nb-Ta-Mo-W [13][47][48][13,57,58], Nb-Ti-Zr [49][50][51][52][53][54][55][56][59,60,61,62,63,64,65,66] and Nb-Ti-V [57][58][59][60][61][67,68,69,70,71].
Near-equiatomic WNbMoTa with a single-phase BCC structure was first proposed by Senkov [13], where the cast sample density and Vickers microhardness were ρ = 13.75 g/cm3 and Hv = 4455 MPa, respectively. The alloy possessed a compressive yielded strength of 1058 MPa [49][59] failed by splitting at εp
) of TaNbHfZrTi HEA [49][59] is superior to the SYS of the TaNbMoW alloy in the temperature range of 296~1073 K (929~535 MPa). When the test temperature exceeds 1073 k, the yield strength of the TaNbHfZrTi HEA with a much lower melting temperature decreases rapidly. Yao et al. [58][68] calculated and designed NbTaTiV, NbTaVW and NbTaTiVW with a single phase, and the results showed that NbTaTiV exhibits exceptional compressive ductility (~50%) at room temperature and a yield strength of 965 MPa, while NbTaTiVW and NbTaVW show yield strengths of 1420 MPa and 1530 MPa with fracture strains of 20% and 12%, respectively (Figure 4). Work hardening can be observed in these HEAs.
The alloying method is also widely used to enhance the mechanical properties of HEAs. The refractory elements (Nb, V, and Zr), Ti and Al with low density are the most commonly used alloying elements to improve the specific yield strength. Ti elements and their alloys possess good ductility and excellent high-temperature properties [63][73]. For example, the solid-solution hardening effect of Ti addition is beneficial to the compressive strength and ductility of NbMoTaW and VNbMoTaW HEAs at room temperature. The yield strengths of TiNbMoTaW and TiVNbMoTaW are ~586 and ~659 MPa at 1200 °C (Figure 5), respectively [63][73], so they are expected to be used as materials for high-temperature applications. At room temperature, the compressive yield strengths of Ti20Zr20Hf20Nb20V20 and Ti20Zr20Hf20Nb20Cr20 as-cast alloys [55][65] are 1170 and 1375 MPa, respectively. Compared with the addition of V, the addition of Cr will form Laves phase precipitation in the BCC matrix and achieve strengthening. Besides this, the structural and mechanical properties of two HEAs remained stable after a short period (10 min) of heat treatment at a high temperature, as shown in Figure 6. Substituting Hf and Cr in the CrMo0.5NbTa0.5TiZr and HfNbTaTiZr HEAs can reduce these alloys’ density and improve the RT strength and ductility [51][61].
In addition to the RHEAs mentioned above, there are few other HEA systems. For example, TiAlFeCoNi HEA [63][73] with the L21-BCC crystal structure was prepared by arc-melting and further processed by the high-pressure torsion (HPT) method. Moreover, the alloy exhibited ultrahigh hardness (880 Hv), low elastic modulus (123–129 GPa) and superior activity for cell proliferation.
2.1.2. Processing by Powder Metallurgy
These samples in the reports mentioned above were prepared by using the vacuum arc-melting technique. Mechanical alloying (MA) with spark plasma sintering (SPS) [64][65][66][74,75,76], as a typical technology of PM, can readily fabricate bulk high-density HEAs with ultrafine grains, excellent microstructural homogeneity, improved strength and hardness. Nb25Mo25Ta25W25 and Ti8Nb23Mo23Ta23W23 HEAs [67][77] were successfully prepared by MA with SPS technology. The compressive yield stress and fracture strain of Nb25Mo25Ta25W25 HEAs with average grain sizes ~0.88 µm are 2460 MPa and 16.8%, which are remarkably superior to those prepared by casting [47][57]. However, coarse grain size is conducive to the improvement of high-temperature strength, which is mainly attributed to the fact that the grain boundary is the weak area and plays as the flow unit at high temperatures [68][78]. Besides this, the addition of Ti can facilitate the grain refinement and Ti particles distributed at grain boundaries can improve the toughness of the Ti8Nb23Mo23Ta23W23 HEA (2377 MPa, 26.3%). The WNbMoTaV HEA [69][79], when sintered, shows an ultra-high compressive yield strength of 2612 MPa with a failure strain of 8.8% at room temperature, respectively. Meanwhile, Long et al. [70][80] reported that Laves phase precipitated in the BCC matrix of NbMoTaWVCr HEA and enhanced its mechanical properties by introducing Cr to the WNbMoTaV HEA. Laves phase formed in the BCC matrix due to the smaller atomic radius of Cr compared with the other alloy elements in those RHEAs [50][54][71][60,64,81]. The compressive yield strength (3416 MPa) of the bulk NbMoTaWVCr HEA is dramatically higher than those of the previously reported refractory HEAs fabricated by casting and powder metallurgy methods, as shown in Figure 7. The outstanding mechanical properties of the WNbMoTaV HEA were attributed to fine-grain strengthening, intrinsic and interstitial solid solution strengthening and Orowan strengthening. The enhancement in yield strength of the NbMoTaWVCr HEA may result from the combined effects of finer grain size, a homogeneous microstructure and enhancement of atomic size misfit caused by the addition of Cr and interstitial solid solution strengthening from O (O was inevitably introduced into the mechanically alloyed powders).
In addition to the WNbMoTa HEA, BCC-structured HEA systems prepared by MA with SPS technology also include AlFeTiCrZnCu [72][73][74][82,83,84], CrTiVTaW [19][75][19,85], TiNbTaZr [38] and FeCrMnV [17][76][77][17,86,87]. The AlFeTiCrZnCu HEA [74][84] prepared by MA with SPS technology can achieve a 99% density and homogeneous nanostructure (~10 nm) and its hardness can reach 2 GPa.
The effect of Ti on the phase structure and mechanical properties of TixWTaVCr HEA [75][85] was studied: a single BCC solid solution could be formed when the Ti content was up to 7%. Compared with pure W and several other HEAs, Ti7WTaVCr has higher room-temperature and high-temperature compressive yield strengths, owing to solid solution strengthening and the effects of Cr and V.
Cao et al. [38] successfully prepared TiNbTa0.5Zr and TiNbTa0.5ZrAl0.2 HEAs with a single BCC phase using powder metallurgy technology. The compressive yield strength and strain for TiNbTa0.5Zr and TiNbTa0.5ZrAl0.2 alloys at room temperature were 1310 MPa, 30% and 1500 MPa, 30%, respectively. Figure 8 compares the compressive properties of these HEAs with those of available refractory HEAs. TiNbTa0.5Zr and TiNbTa0.5ZrAl0.2 present a good combination of strength and plasticity, while most refractory HEAs still follow the strength-ductility trade-off. Besides this, TiNbTa0.5Zr and TiNbTa0.5ZrAl0.2 HEAs show a compressed maximum engineering strain of 50% without any cracking or fractures at 800 °C. Moreover, the NbTaTiV HEA exhibits a compressive yield strength of 1.37 GPa and a high fracture strain of 23% at room temperature. When deformation occurs at 1000 C, it still exhibits a high yield strength of 437 MPa with a compression strain of over 40%. Its outstanding mechanical properties are mainly attributed to the homogeneous and fine microstructures and solid solution strengthening effect.
Figure 8. Compressive properties of the refractory high-entropy alloys obtained at room temperature; compressive yield strength vs. compressive ductility (Reprinted with permission from ref. [38]. Copyright 2018 Elsevier).
The volume fraction of the BCC2 phase gradually increased with the increase in the Al concentration in AlxCrFeMoV HEAs [78][88]. The improvement of compressive yield strength from 2730 to 3552 MPa can be attributed to the solid solution strengthening of Al caused by the appearance of the BCC2 phase. The addition of Al dually influenced the properties of the CrFeMoV alloy by improving its strength and reducing the density of the system. The yield strength and hardness as a function of density were compared with data for previously reported HEAs (Figure 9). AlxCrFeMoV HEAs with outstanding mechanical properties, a low cost and low density, which are better than those of any previously reported HEAs, suggested a promising future for the HEAs in many structural applications.
In another case, new phases appeared in the BCC matrix, such as the B2 phase, HCP phase and FCC phase, after SPS processing. As reported [17], AlCuFeMnTiV HEA prepared by sintering powder containing only the BCC phase has the B2 phase, HCP phase and Cu-rich FCC phase precipitated at the grain boundary in addition to the BCC matrix. It exhibits the best comprehensive mechanical properties, with a density of 6.28 g/cm3, compressive yield strength of 2060 MPa and plastic strain of 15.83%, which are superior to most LWHEAs and traditional lightweight alloys. The high strength and good plasticity of AlCuFeMnTiV HEAs are attributed to the strengthening effect of nano twins precipitated in the FCC phase on grain boundaries.
The above discussion demonstrates that powder metallurgy is a promising way of preparing ductile RHEAs with outstanding comprehensive mechanical properties.
2.1.3. Processing by Additive Manufacturing
Recently, several attempts have been made to prepare BCC-structured HEAs (mainly RHEAs) by laser deposition techniques, and there have been many studies on the preparation of those HEAs by arc-melting and PM. The attempts to prepare BCC structure HEAs using AM technology have mainly been based on DLD technology, and few reports have been published on SLM or SEBM.
The process of DLD may also be known as laser metal deposition (LMD), direct metal deposition (DMD), laser engineered net shaping (LENS) and laser cladding. The MoNbTaW RHEA [79][89] with single-wall structures was the first BCC-structured alloy system to be prepared by the DLD method. The case indicated that it is feasible to fabricate a RHEA through in situ alloying of a Mo-Nb-Ta-W elemental mixture even if cracks appear during processing. The bulk of the crack-free TiZrNbTa RHEAs [80][90] was successfully produced by in-situ alloying of elemental powders using the DLD method. It is one of the few alloy systems with suitable room temperature plasticity among the RHEAs. A well-defined compositional gradient with good hardness (440 HV0.1) was obtained by optimizing the DLD method, as shown in Figure 10.
SLM, as a typical AM technology, can three-dimensionally (3D) fabricate components with intricated shapes and refined resolutions [81][82][83][91,92,93]. The fabrication of BCC-structured HEAs through the SLM process has rarely been considered. A MoNbTaW refractory HEA was prepared via the SLM process using blended elemental powders [45][84][45,94]. There was a deviation between the chemical composition of the prepared sample and that of a pre-mixed powder. This was likely due to surface evaporation of the lower melting-point elements which floated to the upper surface of the melt pool during the SLM process. Composition partitioning was in direct contrast with that reported by Dobbelstein et al. [73][83] for MoNbTaW HEAs fabricated via DLD.
The operating principle was similar to that of SLM, but SEBM used an electron beam instead of a laser beam as the heat source, which meant it has attracted extensive attention in recent years. SEBM has the unique characteristics of a high energy density of the incident electron beam, high scan speed, and moderate operation cost and so on. In addition, the high temperature preheating (up to 1100 °C) of the powder bed by the electron beam prior to scanning and melting is another distinct working condition of SEBM. This working condition results in low residual stress of the built products, making the SEBM process suitable to fabricate complex shaped products and reducing the thermal cracking and distortion of the printed-HEAs [85][86][87][95,96,97].
Hiroshi et al. [86][96] investigated the microstructures and mechanical properties of equiatomic AlCoCrFeNi HEA samples fabricated by SEBM comparing them with those of samples prepared by arc-melting. The proportion of the FCC phase (precipitated at the grain boundaries of the B2/BCC grains) for the bottom was much higher than that for the top. Therefore, the hardness of the SEBM samples gradually decreased with an increased proportion of the FCC phase, and they exhibited much higher plastic deformability than the cast specimen, without a significant loss of strength (Figure 11). Wang et al. precipitated CoCrFeNiMn HEA via EBM, the tensile properties of which (average YS ~205 MPa, elongation ~63%) were almost the same as those of the as-cast form obtained by He et al. [88][98]. The Al0.5CrMoNbTa0.5 HEAs [43] from elemental powder blends were prepared using the SEBM technique. By optimizing the process parameters, the porosity was reduced to replace the post-treatment in the traditional processing technology. However, the simultaneous handling of several elemental or pre-alloyed powders brings new challenges to the deposition process.
2.1.4. Self-Sharpening
The previous three parts of the article mainly summarized the BCC-structured HEAs prepared by three forming methods, largely focusing on their tensile or compressive properties under quasi-static conditions, while it is the dynamic tensile or compressive properties that affect its self-sharpening.
“Self-sharpening” is the ability of the material to maintain its acute head shape during penetration, which is a necessary property of material during armor-piercing [89][99]. HEAs possess a good combination of strength and ductility, which is the premise of excellent self-sharpening. Besides this, high susceptibility to adiabatic shear banding (ASB) is the fundamental cause of self-sharpening behavior that benefits penetration performance [90][91][100,101]. ASB is the dominant deformation mechanism for materials consisting of metals or alloys under high-strain-rate loading, which exhibits a narrow band where large shear deformation occurs in a very short time [92][93][94][95][102,103,104,105].
At present, the research on self-sharpening mainly focuses on W-based alloys [96][97][106,107]. W-based alloys are promising candidates for kinetic energy penetrators because of their high density, strength, and ductility [98][99][108,109]. However, the low susceptibility of W-based alloys to ASB reduces their penetration depth. Thus, it is necessary to develop a new matrix material to replace W-based alloys. The sluggish diffusion and lattice distortion effect of HEAs can achieve a balance between strength and toughness and improve the penetration ability [100][110]. This means that HEAs can be used as potential materials with excellent self-sharpening properties. However, only limited success has been achieved in the development of new tungsten HEAs. A new chemical-disordered multi-phase tungsten HEA (WFeNiMo) was developed by Liu et al. [89][99]. Compared with conventional W-based alloys, WFeNiMo consists of a BCC dendrite phase and a rhombohedral μ phase embedded in the continuous FCC matrix, which means it exhibits outstanding self-sharpening capability, as shown in Figure 12. This is due to the precipitation of the ultra-strong μ phase of WFeNiMo, which can mediate the shear banding by triggering dynamic recrystallization softening. Subsequently, Chen et al. [96][106] conducted experiments with WFeNiMo HEA and W-based alloy projectiles penetrating medium-carbon steel by using a ballistic gun and a two-stage light-gas gun. As the impact velocity increased (1330 m/s~1531 m/s.), the penetration mode of the WFeNiMo HEA projectile changed from self-sharpening to mushrooming.
In general, BCC structure HEAs can be successfully fabricated by arc-melting and PM (MA with SPS technology), but the preparation of pre-alloyed powder is still one of the difficult problems for the attempt of AM technology. BCC structure HEAs have ultrahigh strength, high hardness and room temperature plasticity, which has been gradually improved in recent studies. It has great potential in aerospace, military (high performance penetrator materials) and biomedical fields in future applications.
Figure 12. (a) Compressive stress-strain curves of alloys under quasi-static and dynamic conditions, with macroscopic fracture samples in the inset; (b) Depth of penetration of WFeNiMo rod and 93 W rod versus kinetic energy per volume calculated by ρν2/2
, with photographs of the retrieved remnants, respectively; Longitudinal sections of medium carbon steel targets impacted by (c) a WFeNiMo penetrator and (d) a 93 W penetrator, with SEM micrographs of the remnant in the corresponding insets, respectively (Reprinted with permission from ref. [89][99]. Copyright 2020 Elsevier).
